This is an implementation of the Unify approach for the simplepoly
function API, which wraps the full function API. It is unique in that a
lot of different functions use it, and it is easy to build functions
with it. It needs to use exclusives to represent the different options,
but at least it filters out any that aren't viable.
The Unify implementation here is fairly similar to the patterns in the
template() function.
To improve the filtering, it would be excellent if we could examine the
return type in `solved` somehow (if it is known) and use that to trim
our list of exclusives down even further! The smaller exclusives are,
the faster everything in the solver can run.
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